In the prosthetic art it is well-known to provide a leg prosthesis having a single-axis joint which is lockable in the fully extended leg position to prevent flexure during walking or standing. In the event that the wearer desires to bend the leg to sit down or the like, the joint must be manually unlocked. Similarly, should the wearer wish to rise from a sitting position, the knee joint must be locked immediately after rising, as soon as the leg is fully extended. These locking and unlocking procedures usually require the wearer to bend from the waist and manually actuate a lever or toggle in the hinge lock. Such a task is awkward and is a persistent nuisance. Although more automatic knee joints are available, their cost is prohibitive for many amputees. Thus an economical device for modifying the simple single-axis knee joint to provide easier locking and unlocking thereof is highly desirable.